The invention relates to a circuit configuration for generating a virtual ground as a common reference potential for an exhaust gas probe in a motor vehicle.
As environmental awareness is increasing and resulting exhaust gas regulations are becoming increasingly strict, the need to reduce pollutants in exhaust gases of internal combustion engines in motor vehicles is becoming increasingly important. Compliance with currently valid emission limits for pollutants such as carbon monoxide (CO), nitrogen oxides (NOx) and hydrocarbons (HC) requires, on the one hand, selective engine control and, on the other hand, catalytic post treatment of the exhaust gases. For both measures it is necessary to get measurement values with exhaust gas probesxe2x80x94for example lambda probes or NOx probes.
It is known to use thick film sensors to measure the concentration of pollutants in the exhaust gas of an internal combustion engine. Such a sensor is described, using the example of a NOx, sensor, by N. Kato et al. in the publication xe2x80x9cPerformance of Thick Film NOx, Sensor on Diesel and Gasoline Enginesxe2x80x9d, Society of Automotive Engineers, Publication 970858, 1997. This NOx sensor has two measuring cells and three oxygen pump cells and implements the following measuring concept: in a first measuring cell to which the gas to be measured is fed via a diffusion barrier, a first oxygen concentration is set through the use of a first oxygen ion pump current, wherein no decomposition of NOx, occurs. In a second measuring cell, which is connected to the first measuring cell via a diffusion barrier, the oxygen content is further reduced through the use of a second oxygen ion pump current and NOx. is decomposed at a measuring electrode. The oxygen which is generated in this way is used as a measure of the NOx, concentration. The entire NOx, sensor is heated to an increased temperature, for example 700xc2x0 C., through the use of an electric heating element.
In order to operate such a sensor it is necessary to regulate the respective pump current for the oxygen pump cells precisely. Because motor vehicles generally have an asymmetrical supply voltage, the generation of both positive and negative pump currents requires a reference potential in the center of the supply voltage range.
It is accordingly an object of the invention to provide a circuit configuration which can generate a virtual ground as a common reference potential.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for generating a virtual ground for an exhaust probe operating according to a principle of a galvanic oxygen concentration cell with a solid electrolyte, including:
a micro-controller for determining an actual value of a virtual ground and generating a pulse-width-modulated signal;
an analog circuit connected to the micro-controller, the analog circuit converting the pulse-width-modulated signal into the virtual ground; and
a read-only memory operatively connected with the micro-controller for providing a setpoint value of the virtual ground.
A micro-controller in conjunction with an analog circuit is used to generate the virtual ground. The actual potential value of the virtual ground is read into the micro-controller using A/D converters and compared with a predefined setpoint value. The potential of the virtual ground is regulated to a value in the center of the supply voltage range on the basis of the resulting difference value so that both positive and negative pump currents can be generated. The measuring error of the entire circuit configuration is thus reduced to the errors of the A/D conversion caused by leakage currents and quantization errors.
With the objects of the invention in view there is also provided, an exhaust probe configuration, including:
an exhaust probe having a galvanic oxygen concentration cell configuration with a solid electrolyte;
a circuit configuration connected to the exhaust probe for generating a virtual ground;
the circuit configuration including a micro-controller, an analog circuit and a read-only memory;
the microcontroller determining an actual value of the virtual ground and generating a pulse-width-modulated signal;
the analog circuit converting the pulse-width-modulated signal into the virtual ground; and
the read-only memory providing a setpoint value of the virtual ground.
According to another feature of the invention, the read-only memory is a programmable read-only memory.
According to yet another feature of the invention, the read-only memory is integrated into the micro-controller.
According to a further feature of the invention, the micro-controller includes a difference former, a pulse-width modulation unit, and a controller; the difference former forms a difference value from the actual value of the virtual ground and the setpoint value of the virtual ground; the pulse-width modulation unit generates the pulse-width-modulated signal; and the controller controls the pulse-width modulation unit based on the difference value.
According to another feature of the invention, the controller is a PID controller.
According to yet another feature of the invention, the micro-controller has an output impedance; the analog circuit includes a filter circuit and an impedance converter; the filter circuit converts the pulse-width-modulated signal into a DC voltage signal; and the impedance converter adapts the output impedance of the micro-controller.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for generating a virtual ground as a common reference potential for an exhaust gas probe in a motor vehicle, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.